Conventional medical x-ray imaging devices are based on the attenuation through photoelectric absorption of the x-rays penetrating the object to be imaged. However, for soft tissues including vessels, cartilages, lungs, and breast tissues with little absorption, this provides poor contrast compared with bone images. This problem of low contrast in soft tissues can be addressed with phase contrast imaging (PCI) techniques.
The principle of PCI is based on the wave nature of x-rays, where refraction and diffraction properties need to be considered. As an electromagnetic wave, the x-ray is usually characterized by its frequency, amplitude, and phase. When an electromagnetic wave penetrates a medium, its amplitude is attenuated and its phase is shifted. In x-ray technology, the refractive index n of a material can be expressed by a complex numbern=1−δ+iβ  (1)
The imaginary part β contributes to the attenuation of the amplitude and the real part δ is responsible for the phase shift. It has been shown that δ is about 103 to 104 times larger than β. But in conventional medical imaging, only the information of β is recorded while the information of δ is completely lost. In recent years, several PCI techniques have been explored to make use of the phase shift to form the image, which is expected to provide more information about the object. Additionally, the diagnostic capabilities can be improved if spectral imaging techniques were implemented. So far, spectral imaging, which implies the x-ray acquisition at different mean energies of x-ray spectra (e.g., dual energy technique) have been primarily utilized in conventional absorption type of imaging.